The present invention relates to a method and system for use in assessing collateral blood flow to a tissue region of interest, and is particularly apt for implementation in photoplethysmographic systems.
The assessment of collateral blood flow to a specific bodily tissue region is of interest to medical personnel in a variety of situations. In this regard, medical personnel may be interested in assessing the adequacy of blood flow to a tissue region by a first supply artery when a second supply artery is not supplying blood thereto. Typically, such an assessment may be important when assessing the viability of relying upon a secondary artery to supply adequate blood flow to a tissue region of interest in the event the primary artery thereto is occluded or otherwise unable to deliver blood in conjunction with or pursuant to a given medical procedure.
By way of example, collateral blood flow to the hand through the radial artery is of interest when assessing the feasibility of utilizing the radial artery for cannulation or for harvesting graft tissue in conjunction with a given medical procedure (e.g., heart bypass surgery). To date, such assessments are often made via a manual test known as an xe2x80x9cAllen""s Testxe2x80x9d. In the Allen""s Test, the patient tightly clenches a fist and then the clinician applies pressure to the radial and ulnar arteries to occlude further blood flow to the hand under test. The patient then opens the clenched fist while pressure is maintained over both arteries. The hand appears blanched and free of blood. Pressure on the ulnar artery is released and the time required for blood flow to reach each finger is noted by measuring the time required for the color in the fingertips to change back from the blanched state to the normal blood filled state. The time required for color to return to the fingers is inversely proportional to the adequacy of the collateral blood flow through the ulnar artery.
As may be appreciated, use of the Allen""s Test to assess collateral blood flow involves a significant degree of subjectivity. The present inventors have recognized the desirability of providing a method and system that provides a more objective measure for assessing collateral blood flow.
In view of the foregoing, a general objective of the present invention is to provide an improved approach for assessing collateral blood flow to a tissue region of interest.
A more specific objective of the present invention is to provide an approach for collateral blood flow assessment that is both quantitative and more reliable than currently utilized techniques.
Another objective of the present invention is to provide a non-invasive and at least partially-automated approach for collateral blood flow assessment.
An additional objective of the present invention is to provide an improved collateral blood flow assessment approach that may be readily implemented in photoplethysmographic systems.
The above objectives and additional advantages are realized in an inventive method that includes a first step of obtaining at least a first pulsatility measurement (FPM) and a second pulsatility measurement (SPM) indicative of the volume of blood flow to a tissue region of interest (ROI). For purposes hereof, the term xe2x80x9cpulsatility measurementxe2x80x9d encompasses any technique employable to obtain a measure, relative or otherwise, of the perfusion of a tissue ROI of a patient or test subject. In the inventive method, one of the FPM and SPM is obtained with a plurality of arteries supplying blood to the tissue region of interest, while the other of the FPM and SPM is obtained with at least a selected one of said plurality of said arteries substantially occluded. Following the obtainment of the first and second pulsatility measurements, the method further includes the step of employing such measurements to obtain a collateral blood flow value (CFV) employable by medical personnel to quantify the acceptability of collateral blood flow to the tissue ROI.
Pulsatility measurements may be obtained by passing at least a first light signal through the tissue ROI and detecting the intensity of the transmitted signal. The detected signal may be utilized to compute (e.g. via a processor) the first pulsatility measurement and second pulsatility measurement. For example, where a single light signal is utilized the pulsatility measurements may simply entail the calculation of the percent modulation of the transmitted signal. In another approach, the pulsatility measurements may be perfusion index values obtained in a manner taught by U.S. Pat. No. 5,766,127, hereby incorporated by reference. As will be appreciated, the present invention may be readily implemented in and is otherwise particularly apt for use in a photoplethysmographic system.
In a further aspect, the inventive method may comprise the step of storing the first and second pulsatility measurements (e.g. in memory in a microprocessor based system) for use in obtaining the CFV. By way of example, the CFV may be a computed ratio of the FPM and SPM, expressed as a percentage. Further, the inventive method may comprise the additional step of outputting the CFV to a user via a user interface, e.g. via display and/or audible signals.
The first pulsatility measurement and second pulsatility measurement should be obtained during non-overlapping time periods. Further the inventive method may comprise the step of prompting a user at the beginning and/or ending of the time period associated with the obtainment of the FPM and/or the time period associated with obtainment of the SPM. Such prompting facilitates the establishment of the desired arterial occlusion or non-occlusion for the given pulsatility measurement. By way of example, prompts may be provided in the form of display and/or audible signals. Further, the prompts may comprise instruction information to guide a user through the occlusion steps.
In an additional aspect, obtaining the first pulsatility measurement may entail the steps of obtaining a plurality of first pulsatility values (FPVs) and determining the FPM utilizing the first plurality of FPVs. The method may further include the substep of comparing the plurality of FPVs to a first reference range (e.g. such range being set to be xc2x1 a predetermined percentage of a mean value of the first plurality of first pulsatility values), wherein if/when all of the plurality of FPVs are within the first reference range obtainment of the FPM may be completed. In the later regard, a mean value of the plurality of FPVs may be used as the FPM. In the event that a given plurality of FPVs do not fall within the first reference range, the method may further provide for the obtainment of a new plurality of FPVs and subsequent comparison thereof to a corresponding first reference range. Such substeps may be repeated until a given plurality of FPVs fall within a corresponding first reference range. As may be appreciated, the obtainment of a plurality of FPVs and the comparison thereof to a first reference range yields enhanced accuracy in relation to the FPM based thereupon. In turn, overall reliability is increased.
Correspondingly, the obtainment of a second pulsatility measurement may entail the obtainment of a plurality of second pulsatility values (SPVs) and the comparison of the plurality of SPVs to a second reference range (e.g. such range being set to be xc2x1 a predetermined percentage of a mean value of the plurality of SPVs). When all of the SPVs are within the second reference range obtainment of the SPM may be completed (e.g., by using a mean value of the SPVs). In the event a given plurality of SPVs are not all within a second reference range a new plurality of SPVs may be obtained and compared to a corresponding second reference range. Such substeps may be repeated until a given plurality of SPVs fall within a corresponding second reference range. The obtainment and noted use of a plurality of SPVs yields enhanced accuracy of an SPM based thereupon, and improves overall reliability.
In addition the present invention may include a check procedure for further enhancing the accuracy and reliability of the measurements. In particular, a check pulsatility measurement (CPM) may be obtained after FPM obtainment and prior to SPM obtainment, wherein the CPM is obtained with all of the plurality of arteries supplying the ROI substantially occluded. The CPM may be compared to a check reference range, wherein if the CPM is within the check reference range the SPM may be obtained. By way of example, when the FPM is obtained with all of the plurality of arteries to an ROI open, then if the CPM is within a predetermined small percentage (e.g., 0 to 5%) of the FPM, the SPM may be obtained. In the event the CPM is not within the check reference range a user prompt may be provided. The prompt may be in the form of a displayed or audible signal, and may include remedial action information. Such an approach serves to ensure that the artery which is intended to be occluded during obtainment of an SPM is in fact sufficiently occluded.
The present invention is particularly apt for implementation in a photoplethysmographic system. In our current embodiment of this invention a photoplethysmographic sensor provides an output signal indicative of the amount of a light transmitted through a tissue region of interest. The system includes a means for utilizing the sensor output signal to obtain first and second pulsatility measurements, e.g. an FPM and SPM as noted above. The system calculates the collateral flow from the FPM and SPM, utilizing the CPM procedure as noted above.
The system may be provided for operation in at least two modes, wherein in the first mode of operation the system measures at least one blood analyte concentration value (e.g. blood oxygenation levels), and wherein in the second mode of operation the system provides a CFV value as noted above. In this regard, the system may further include a user interface selection means for allowing a user to select system operation in the first mode or second mode. As may be appreciated, the user interface means may be further operable to output a CFV to a user via display/audible signals. In the later regard, the system may be designed so that the output mode is also selectable by a user.
In conjunction with the inventive system, the pulsatility means may be operable to automatically obtain a first plurality of pulsatility values for automatic computation of an FPM, and to automatically obtain a second plurality pulsatility values for automatic computation of an SPM. Automatic computation of the FPM and SPM may entail the computation of mean values and comparison of individual pulsatility measurements thereto, as discussed above.
The pulsatility means may further provide for the automatic obtainment of a check pulsatility measurement, or CPM as noted above, wherein an SPM is not obtained until a CPM is obtained within a predetermined range. In the event a CPM is not obtained within the check reference range, the user interface means may be provided to provide remedial information to a user. By way of example, such remedial information may instruct a user to relocate the point of pressure application to the arteries supplying blood to the tissue region of interest.
The user interface means of the inventive system may further provide for user prompting in relation to the sequence of occluding and opening the plurality of arteries supplying blood to a given tissue region of interest. More particularly, such prompting may comprise audible/visual output information to assist/instruct medical personnel in occluding the correct arteries. Such prompting may be further provided in a manner so that each occlusion/opening step is only undertaken after a desired degree of accuracy and/or stability is achieved with respect to the FPM, CPM and SPM in a course of a given collateral blood flow test procedure.
Additional aspects and advantages of the present invention will be apparent to those skilled in the art upon review of the further description that follows.